1. Technical Field
The present invention relates to a liquid crystal display panel with multi-domain unit pixels and an optical mask for manufacturing the liquid crystal display panel in a photo-alignment process.
2. Discussion of the Related Art
A liquid crystal display module includes a liquid crystal display panel, on which images are displayed by changing an arrangement of liquid crystal molecules in a liquid crystal layer according to an electrical field generated in the liquid crystal layer, a backlight assembly for providing light to the liquid crystal display panel, and a case in which the liquid crystal display panel and the backlight assembly are fixed.
A liquid crystal display panel includes the liquid crystal layer, a pair of substrates with the liquid crystal layer interposed therebetween, and a pair of polarizers attached to the exteriors of the substrates. It is desirable for an image displayed on a liquid crystal display panel to have the same display quality no matter which direction it is viewed. To this end, many attempts have been made. For example, a Vertical Alignment (VA) mode liquid crystal display, which uses verticality of liquid crystal molecules with respect to a substrate, and a Plane to Line Switching (PLS) mode liquid crystal display, which uses horizontality of liquid crystal molecules with respect to a substrate have been developed. Because the liquid crystal molecules of these displays have similar refractive-index anisotropy characteristics in different directions, VA and PLS mode liquid crystal displays have a wide viewing angle.
For more improved viewing angle characteristics, patterns of metal wires, slits or projections made of an organic film are formed on unit pixels so that liquid crystal molecules may have a similar slope in different directions. However, since liquid crystal molecules are affected by a fringe field, the patterns, the slits or the projections may reduce an aperture ratio, which is a ratio of the region where the light provided from a backlight assembly passes through a unit pixel to the total area of the unit pixel. The term ‘unit pixel’ as used herein may refer to a pixel representing the basic colors of a liquid crystal display panel.
Liquid crystal molecules should always maintain the same arrangement with respect to the same potential. To this end, a pretilt is formed in an alignment film on a substrate to fix a direction and a slope of liquid crystal molecules located near the substrate. The pretilt of an alignment film is formed by physically rubbing a rubbing cloth on an alignment material previously formed on the substrate. However, the method of using the rubbing cloth may reduce the yield of liquid crystal display panels since a foreign substance may be introduced or static electricity may occur on the alignment layer due to the contact. In addition, rubbing cloths are frequently replaced, causing an increase in process time and cost.
To improve yield of a liquid crystal panel, a photo-alignment process has been introduced. The photo-alignment process forms a pretilt of an alignment film using a non-contact method without forming patterns, slits or projections in a pixel region. The photo-alignment process includes applying a photo-reactive material onto a substrate and obliquely irradiating ultraviolet (UV) light to the surface on which the photo-reactive material is applied. A pretilt of an alignment film is formed according to the direction of the irradiation. Accordingly, liquid crystal molecules may be tilted in several different directions by dividing a unit pixel into several regions and irradiating light thereto in different directions.
For example, a unit pixel may be divided in horizontal and vertical directions to have four domains. Liquid crystal molecules are tilted in different directions according to the domains. However, liquid crystal molecules located at boundary parts between neighboring or adjacent domains may not be tilted to correspond to a voltage applied to the pixel. As a result, these liquid crystal molecules may block light, forming a Domain Boundary Texture (DBT) where a normal luminance does not appear in the domain. In other words, the DBT is a dark part or a shadow of a unit pixel, and distinguishes domains whose liquid crystal molecules are tilted in different directions.
A unit pixel has a pixel electrode formed on one substrate, a common electrode formed on another transparent substrate spaced apart from the substrate of the pixel electrode, and a liquid crystal layer interposed between the two substrates. The pixel electrode is formed on each of a plurality of unit pixels arranged on one substrate while the common electrode is formed on the entire surface of another substrate, causing a fringe field to be formed between the edge of the pixel electrode and the common electrode. Liquid crystal molecules, which are influenced by the fringe field, are independently arranged without being affected by a pixel potential, and block the light provided from the backlight assembly, forming a Fringe Field Texture (TFT) where the normal luminance does not appear in the domain.
Directions of pretilts of respective domains are matched to polarization axes of polarizers attached to substrates of a liquid crystal display panel. Since the pretilts of the respective domains are substantially perpendicular to at least one polarization axis, the light which has passed through liquid crystal molecules near the DBTs or the edge of the pixel electrode is not perpendicular to the polarization axes of the polarizers. As a result, the luminance may be locally reduced in the DBTs or in vicinity of the edges of the domains.
An aperture ratio of a unit pixel is calculated by dividing an area of the unit pixel having the normal luminance by the total area of the unit pixel. A light transmittance of a unit pixel is calculated by dividing a luminance at which light has penetrated the unit pixel by a luminance of the backlight assembly before light penetrates the unit pixel. Both the DBT and the FFT lead to a reduction in the luminance of unit pixels, causing a decrease in the aperture ratio and the light transmittance of the multi-domain unit pixels.
In the photo-alignment process, an angle of a pretilt, or a pretilt angle, is determined according to the intensity of the irradiated light and/or the irradiation time. If the pretilt angle is excessively large, the molecules farther from the alignment film may be falsely arranged not to agree with the potential applied to the pixel electrode. As a result, unit pixels may show a luminance higher or lower than the normal luminance, reducing a contrast ratio of the liquid crystal display panel and causing a black afterimage phenomenon in which a gray color appears in the unit pixel when a signal representing a black image is provided to a pixel electrode.